I don't work in gamestop but i have a few friends who do and i'm often "hanging out" in gamestop chatting with them about games helping out and getting to see the games before the hit the shelfs XD somtimes play them on the xbox they have in the store a day or two before the game comes XD
anyway last time someone came in with a psp(not go) it gave what should be around 50$ and since yours a 2000 model it might only give around 40$
handhold games normally don't give very much unless it's special titles or very new games my guess is that transformers will give 5$ while R&C would give 2-3$
i'm not so sure about family guy, the might sell it for around 15-20$ since it might go quite easy.i would expect they would give 1/4 or 1/3 of that buying the umd
my guess would be a total of 50-65$ for all of it
but go down and ask them they can answer you quite fast

beside if you want the money cash they normally take away 20% of it